As mobile devices such as mobile electronic devices and mobile communication devices have highly developed, secondary batteries with higher energy density are recently needed to improve efficiency and reduce the size and weight of the devices.
The capacity of the secondary batteries of this type can be improved by known methods: use of a negative electrode material made of an oxide of V, Si, B, Zr or Sn, or a complex oxide thereof (See Patent Literatures 1 and 2, for example); use of a negative electrode material made of a metallic oxide subjected to melting and rapid cooling (See Patent Literature 3, for example); use of a negative electrode material made of a silicon oxide (See Patent Literature 4 for example); use of a negative electrode material made of Si2N2O and Ge2N2O (See Patent Literature 5 for example), and others.
The negative electrode materials can be made conductive by known methods: performing mechanical alloying of SiO and graphite, and carbonizing the resultant (See Patent Literature 6, for example); coating silicon particles with carbon layers by chemical vapor deposition (See Patent Literature 7, for example); coating silicon oxide particles with carbon layers by chemical vapor deposition (See Patent Literature 8, for example).
Although these conventional methods increase the charging and discharging capacities and energy density to some extent, the increase is insufficient for market needs and the cycle performance fails to fulfill the needs. The conventional methods need to further improve the energy density and thus are not entirely satisfactory.
Patent Literature 4 discloses use of a silicon oxide as a negative electrode material for a lithium-ion secondary battery so as to obtain an electrode with a high capacity. To the present inventor's knowledge, however, this method cannot achieve low irreversible capacity at first charging and discharging and a practical level of cycle performance; this method can be improved on to solve these problems.
The methods to provide a negative electrode material with conductivity remain the following problems. The method in Patent Literature 6 uses solid-state welding and thus cannot uniformly form a carbon coating, resulting in insufficient conductivity.
Although the method in Patent Literature 7 enables the formation of a uniform carbon coating, this method uses Si as a negative electrode material and thus reduces the cycle performance because the expansion and contraction of the material becomes too large at lithium insertion or extraction. This makes the material unsuited to practical use. The charging capacity consequently needs to be limited to avoid this problem.
Although the method in Patent Literature 8 enables the improvement in cycle performance, the precipitation of silicon fine crystals, the structure of a carbon coating and the combination between the carbon coating and the base are unsatisfactory. Thus, the material produced by this method is unpractical for use in secondary batteries. This material causes the batteries to gradually reduce the capacity with an increase in charging and discharging cycles and to greatly reduce the capacity after given cycles.